The present invention relates generally to electrochromic windows and more particularly to a novel, transparent, electrically-conductive, ion-blocking layer suitable for use in electrochromic windows.
It has been estimated that approximately 40% of the annual national energy consumption is used to control the climate of building interiors, i.e. to heat building interiors in cooler months and to cool building interiors in warmer months. It has also been estimated that, of this amount, approximately 33% is wasted, primarily due to the loss of radiation through building windows. For example, radiation loss occurs on warm days as solar energy is transmitted into building interiors through windows, causing the building interiors to be warmed and, consequently, requiring additional energy to be expended for cooling. Additionally, radiation loss occurs on cool days as thermal infrared radiation present within building interiors escapes through building windows, thereby requiring additional energy to be expended as heat to warm the interiors.
One proposed solution to the problem of radiation loss has involved the use of electrochromic windows having variable absorption. The idea behind such windows is that, by absorbing solar energy, the windows prevent solar energy from entering a building interior and, therefore, from heating the building interior. Unfortunately, however, such windows frequently become very hot as a result of absorbing solar energy. Moreover, the windows ultimately re-radiate approximately 50% of the absorbed energy into or out of the building interior, resulting in thermal transfer inefficiencies.
A second proposed solution to the problem of radiation loss has involved the use of electrochromic windows having variable reflectance over a broad spectral bandwidth of radiation. Using such windows, it is possible, for example, on warm days to transmit the visible portion of the solar spectrum so as to illuminate a building interior while reflecting the ultraviolet and infrared components of the solar spectrum so as to decrease the cooling load and, additionally, on cool days to transmit into the building interior, the entire solar spectrum so as to both illuminate and heat the building interior while reflecting back into the building interior thermal infrared radiation already generated therewithin. Examples of variable reflection electrochromic windows are described in U.S. Pat. Nos. 4,889,414, 4,832,463, 4,876,628 and 5,171,413, all of which are incorporated herein by reference.
According to the teachings of the aforementioned patents, a variable reflection electrochromic window typically comprises a transparent substrate and a thin film, multilayer coating whose transmissivity is adjustable by the transport of electrons and ions therethrough. Typically, the coating includes five layers deposited directly on top of one another, the first layer being a transparent, electrically-conductive layer, the second layer being an electrochromic layer having a spectral selectivity which is adjustable, the third layer being an ion-conductive, electron-resistive layer capable of reversibly transporting positive metal ions into and out of the electrochromic layer so as to transform said electrochromic layer to and from an optically reflective state, the fourth layer being a counter-electrode layer capable of donating and receiving electrons and ions to and from said electrochromic layer, and the fifth layer being a transparent, electrically-conductive layer. As can readily be appreciated, the first and fifth layers function primarily as transparent electrical contacts for dispersing electrons over the surfaces of the second and fourth layers, respectively, and can be omitted from the device, if desired.
Preferably, the first and fifth (i.e., electron conductor) layers are formed from indium tin oxide (ITO); the second (i.e., electrochromic) layer is formed from WO.sub.3 ; the third (i.e., ion-conductor) layer is formed from Li.sub.2 O:Nb.sub.2 O.sub.5 ; and the fourth (i.e., counterelectrode) layer is formed from LiCoO.sub.2. When an externally-generated electric field of the proper polarity is applied to the multilayered structure, lithium ions migrate from the LiCoO.sub.2 layer to the WO.sub.3 layer and become incorporated into the polycrystalline structure of WO.sub.3. The incorporation of lithium ions into the polycrystalline structure of WO.sub.3 causes that layer to become "colored," i.e., reflective of certain wavelengths of radiation. In a complementary fashion, the removal of lithium ions from LiCoO.sub.2 causes that layer also to become "colored."
Typically, all five of the above-described layers are deposited by a sputtering technique, such as by rf diode sputtering. Problems associated with the deposition of certain of these layers, and refinements which have been to overcome these problems are discussed in several of the above-identified U.S. patents.
Another type of problem associated with variable reflection electrochromic windows of the type described above is that, upon the application of a voltage required for electron transfer, the positive (e.g., lithium) ions have a tendency to migrate from the electrochromic and counter-electrode layers to the outer, electron conductor (ITO) layers and/or to irreversibly react with atmospheric gases surrounding the device, thereby causing a depletion of the necessary quantity of lithium ions in the device. Thus depleted, the electrochromic and counter-electrode layers are prevented from functioning in their intended manner.
One approach to this problem has been to insert transparent, electrically-conductive, ion-blocking layers between the first (ITO) and second (electrochromic) layers and the fourth (counter-electrode) and fifth (ITO) layers of the above-described multilayer coating. This approach, which is described in U.S. Pat. No. 5,133,594, also incorporated herein by, reference, is said to involve using zinc oxide, Cadmium oxide or silicon carbide as the transparent, electrically-conductive, ion-blocking layers.
Although the present inventors have found that zinc oxide, cadmium oxide and silicon carbide, when used in the manner described in U.S. Pat. No. 5,133,594, are generally satisfactory at blocking the migration of lithium ions, the present inventors have also found that these layers are not sufficiently electrically-conductive, thereby resulting in electrochromic windows with very poor transmissivity switching behavior--very slow switching and/or relatively high voltages needed for switching.